EF Core

The provided code snippet demonstrates a LINQ query to group by multiple columns while also finding the maximum item ID within each group. Here's a breakdown of the code:

var result = from o in context.Orders
             group o by new { o.CustomerId, o.OrderType } into g
             select new
             {
                 CustomerId = g.Key.CustomerId,
                 OrderType = g.Key.OrderType,
                 MaxItemId = g.Max(x => x.ItemId)
             };

In this code:

• The group by new { o.CustomerId, o.OrderType } syntax allows grouping by a combination of CustomerId and OrderType using an anonymous type
• The into g clause signifies that the grouped data will be accessible through the identifier g
• The g.Key property allows access to the grouped key values, such as CustomerId and OrderType.
• select new { ... } creates a new anonymous object for each group containing CustomerId, OrderType, and the maximum ItemId
• The Max() method is used to find the maximum ItemId within each group (g.Max(x => x.ItemId))

This code efficiently retrieves the maximum ItemId for each unique combination of CustomerId and OrderType in the orders collection.

In EF Core, joining a subquery with two columns (INNER JOIN table) can be achieved using LINQ syntax. Below is an example of how to do this:

var query = from user in context.Users
            join post in context.Posts
            on new { UserId = user.Id, IsPublished = true }
            equals new { post.UserId, IsPublished = true }
            select new
            {
                user.Username,
                post.Title
            };

In this example

• The LINQ query joins the Users and Posts tables on two columns (UserId and a condition IsPublished) using the equals keyword.
• In a LINQ join operation, you need to match corresponding columns or properties from the joined tables/entities. In the case of UserId = user.Id, you are matching the UserId column from the Post entity with the Id column from the User entity.

Additional reading at EF Core Join Query - TekTutorialsHub

To join a subquery (INNER JOIN table) in EF Core, you can use the Join method along with LINQ expressions. Below is a example code snippet:

var query = from order in context.Orders
            join orderItem in context.OrderItems
            on order.OrderId equals orderItem.OrderId
            where order.CustomerName == "John Doe"
            select new
            {
                order.OrderId,
                order.CustomerName,
                orderItem.ProductName,
                orderItem.Price
            };

The expression on order.OrderId equals orderItem.OrderId is used to specify the join condition between two tables/entities (Orders and OrderItems  based on their related columns OrderId.

Additional Reading at EF Core Inner Join (csharptutorial.net)

Entity Framework Core (EF Core) does wrap the SaveChanges method in a transaction by default. When you call SaveChanges to persist changes to the database, EF Core ensures that all the changes are committed as a single transaction. This means that if any part of the operation fails (e.g., due to a validation error or a database constraint violation), none of the changes will be applied to the database.

Here's how it works:

1. You make changes to your entity objects within a DbContext.
2. When you call SaveChanges, EF Core starts a database transaction.
3. EF Core applies all the changes to the database within this transaction.
4. If all changes are successfully applied, the transaction is committed, making the changes permanent.
5. If any part of the operation fails (e.g., an exception is thrown), the transaction is rolled back, and no changes are applied to the database.

This behavior ensures that your data remains in a consistent state, and either all changes are applied or none are. If you need more control over transactions, such as specifying isolation levels or manually managing transactions, EF Core provides options for doing so. You can use methods like BeginTransaction, Commit, and Rollback on the DbContext's Database property to work with transactions explicitly.

In this example, you can see how you can manually manage a transaction around your database operations, providing more fine-grained control when needed. However, for most scenarios, the default behavior of wrapping SaveChanges in a transaction is sufficient.

using (var dbContext = new YourDbContext())
{
    using (var transaction = dbContext.Database.BeginTransaction())
    {
        try
        {
            // Perform your database operations here

            dbContext.SaveChanges();

            // If everything is successful, commit the transaction
            transaction.Commit();
        }
        catch (Exception ex)
        {
            // Handle exceptions and optionally roll back the transaction
            transaction.Rollback();
        }
    }
}

In this example, you can see how you can manually manage a transaction around your database operations, providing more fine-grained control when needed. However, for most scenarios, the default behavior of wrapping SaveChanges in a transaction is sufficient.

Make EF Core update only the updated properties by turning the disconnected scenario to connected.

public void SaveBook(Book book)
{
    // Here, 'book' is the book with the changed Title.
    using(var context = new TestContext())
    {
        var dbBook = context.Books.Find(book.ID);

        // Copy book's property values to dbBook.
        context.Entry(dbBook).CurrentValues.SetValues(book);

        context.SaveChanges();
    }
}

There may be good reasons to prefer the latter method above the former.